1. Field of the Invention
The present invention relates to an ignition coil and, more particularly, to an ignition coil which has a magnetic core and primary and secondary windings wound around the magnetic core, at least the secondary winding being wound on a tubular secondary bobbin and the primary winding being arranged in the secondary bobbin.
2. Discussion of Related Art
Such ignition coils are well known in the field of internal combustion engines. When a low voltage is applied and then removed at the terminals of the primary winding, this results in a high voltage pulse at the terminals of the secondary winding. This pulse is applied to an ignition coil of the engine, which causes the explosion of the gases contained in the corresponding cylinder.
Generally, the primary winding is itself wound on a primary bobbin arranged within and coaxial to the second bobbin. However, it is also possible to wind the primary winding directly on the magnetic core.
In the case of an ignition coil of the so-called single terminal type, one of the ends of the secondary winding is connected to ground, that is to say, in practice to one of the ends of the primary winding, and the other end of this secondary winding which constitutes the high voltage terminal is connected to the spark plug. A difficulty arises on the part of the coil where this high voltage terminal is located. In fact, at the time of the pulse it is brought to a voltage which is several tens of a kilovolt with respect to the ground and, in fact, with respect to the entire primary winding since only a low voltage is present at the terminals of the latter. Accordingly, there are important risks of disruptive breakdown between the high voltage terminal and the primary winding.
In EP-A-0 071 172, a secondary bobbin is proposed which is closed on the side of the high voltage terminal end of the secondary winding.
In that document the secondary bobbin is in the form of a tube provided with a bottom on the side of the end of the secondary winding forming the high voltage terminal. This bobbin can be made of a plastic material of high dielectric strength, making the forming of an arc between the high voltage terminal and the primary winding impossible.
So-called double terminal coils are also known. In such coils, each of the two ends of the secondary winding is connected to a spark plug. The high voltage pulse is formed at the time that the cylinder corresponding to one of the spark plugs contains the mixture to be exploded and the cylinder corresponding to the other spark plug contains burned gases. Due to the differences in electric properties between the combustible mixture and the burned gases, a single spark of high energy is formed on the side of the combustible mixture, the other spark, or lower energy, serving only as ground return.
The problem of disruptive breakdown at the place of the high voltage terminal arises therefore differently in the case of a double terminal spark plug. In fact, with an arrangement such as that which has just been described, one of the high voltage terminals would be effectively insulated from the primary winding by the bottom of the secondary bobbin, but this would not be true of the side of the other high voltage terminal, where disruptive breakdowns could therefore take place.